Britain on Saturday denounced Iran's seizure of a British-flagged oil tanker in the Gulf as a "hostile act" and rejected Tehran's explanation that it seized the vessel because it had been involved in an accident.

Iran's Revolutionary Guards posted a video online showing speedboats pulling alongside the Stena Impero tanker, its name clearly visible. Troops wearing ski masks and carrying machine guns rappelled to its deck from a helicopter, the same tactics used by British Royal Marines to seize an Iranian tanker off the coast of Gibraltar two weeks ago.

Friday's action in the global oil trade's most important waterway has been viewed in the West as a major escalation after three months of confrontation that has already taken Iranian regime and the United States to the brink of war.

It follows threats from Tehran to retaliate for Britain's July 4 seizure of the Iranian tanker Grace 1, after it violated sanctions on Syria.

British Defence Secretary Penny Mordaunt called the incident a "hostile act". Foreign Secretary Jeremy Hunt said he had expressed "extreme disappointment" by phone to his Iranian counterpart, Mohammad Javad Zarif. Britain also summoned the Iranian charge d'affaires in London.

A spokesman for Iran's Revolutionary Guards, Brigadier-General Ramezan Sharif, said Tehran had seized the ship in the Strait of Hormuz despite the "resistance and interference" of a British warship which had been escorting it. No British warship was visible in the video posted by the Guards.

Iranian regime's Fars news agency said the Guards had taken control of the Stena Impero on Friday after it collided with an Iranian fishing boat whose distress call it ignored.